Conventionally, linear motors have been used as sources of reciprocating vibrations (refer to Japanese Published Patent Application No. Hei.2-52692 (FIG. 1)). This literature discloses a linear motor for generating a reciprocating vibration (hereinafter referred to as a linear vibration motor).
The linear vibration motor is constructed as a single-phase sync motor having a mover comprising a rod-shaped permanent magnet, and a stator comprising an electromagnet. The electromagnet is obtained by winding a coil around both ends of a U-shaped iron core.
An output voltage from an AC power supply is converted into a full-wave rectified voltage having a frequency twice as high as a frequency of the AC power supply voltage by a full-wave rectifying circuit. When the full-wave rectified voltage is supplied to the coil of the linear vibration motor, the mover reciprocates to generate vibrations.
When generating vibrations by the reciprocating motion of the mover as described above, a strong magnetic power is needed. In the linear vibration motor, the mover is supported by a spring so as to form a spring vibration system including the mover, and this spring vibration system is vibrated at a frequency (resonance frequency) that matches the natural frequency of the spring vibration system, whereby energy necessary for driving the linear vibration motor can be reduced.
However, in the above-described method of driving the linear vibration motor at a frequency that matches the natural frequency of the spring vibration system, when a load is applied to the linear vibration motor, the amplitude of the reciprocating motion of the mover is not stable.
On the other hand, there has been proposed a method for driving and controlling a linear vibration motor, in which at least one of displacement, speed, and acceleration of a mover of the linear vibration motor is detected, and supply of power to an electromagnetic coil of the motor is adjusted according to a result of detection (refer to Japanese Published Patent Application No. Hei.10-243622 (FIG. 1)). In this method, even when the natural frequency (resonance frequency) varies for some reason such as load fluctuations, power supply to the coil is adjusted so that the linear vibration motor is continuously driven in the resonance state, on the basis of the displacement, speed, or acceleration of the mover.
In the above-described linear vibration motor drive control method, however, the load applied to the linear vibration motor is extremely large, and thereby the amplitude, speed, or acceleration of the mover is significantly reduced. When detection of displacement, speed, or acceleration of the mover with a sensor becomes impossible, the linear vibration motor cannot be driven in the resonance state, resulting in a significant reduction in driving efficiency.
Furthermore, in the linear vibration motor drive control method, since a sensor for detecting displacement of the mover, for example, must be incorporated in the linear vibration motor, the volume of the linear vibration motor is increased by the volume of the sensor, and furthermore, operation reliability of the sensor must be secured under severe operating conditions such as temperature.